R.1.23
Date Code 20111215 Reference Manual SEL-421 Relay
Protection Functions
Loss-of-Potential Logic
Loss-of-Potential Logic
Fuses or molded case circuit breakers often protect the secondary windings of
the power system potential transformers. Operation of one or more fuses or
molded case circuit breakers results in a loss of polarizing potential inputs to
the relay. Loss of one or more phase voltages prevents the relay from
discriminating fault distance and direction properly.
An occasional loss-of-potential (LOP) at the secondary inputs of a distance
relay is unavoidable but detectable. The relay detects a loss-of-potential
condition and asserts Relay Word bits LOP (Loss-of-Potential Detected) and
ILOP (Internal Loss-of-Protection From ELOP Setting). This allows you to
block distance element operation, block or enable forward-looking directional
overcurrent elements, and issue an alarm for any true LOP condition.
If line-side PTs are used, the circuit breaker(s) must be closed for the LOP
logic to detect a three-phase LOP condition. Therefore, if three-phase
potential to the relay is lost while the circuit breaker(s) is open (e.g., the PT
fuses are removed while the line is de-energized), the relay cannot detect an
LOP when the circuit breaker(s) closes again.
The SEL-421 also asserts LOP upon circuit breaker closing for one or two
missing PTs. If the relay detects a voltage unbalance with balanced currents at
circuit breaker close, then the relay declares a loss-of-potential condition.
Inputs into the LOP logic are as follows:
➤ 3PO—three-pole open condition
➤ SPO—single-pole open condition
➤ OOSDET—out-of-step condition detected
➤ OST—out-of-step tripping assertion
➤ V
1
—positive-sequence voltage (V secondary)
➤ I
1
—positive-sequence current (A secondary)
➤ V
0
—zero-sequence voltage (V secondary)
➤ I
0
—zero-sequence current (A secondary)
All three poles of the circuit breaker(s) must be closed (i.e., Relay Word bit
3PO equals logical 0) and neither Relay Word bit OSB nor OST can be
asserted for the LOP logic to operate.
The relay declares an LOP condition (Relay Word bit LOP equals logical 1) if
V
1
drops in magnitude by at least ten percent and there is no corresponding
change in I
1
or I
0
magnitude or angle. An LOP condition persisting for
15 cycles causes the LOP logic to latch. LOP resets (Relay Word bit LOP
equals logical 0) when V
1
returns to a level greater than 85 percent nominal
voltage and V
0
is less than 10 percent of V
1
.
The LOP logic requires no settings other than enable setting ELOP.
Setting ELOP := N
If you set ELOP to N, the LOP logic operates but does not disable any
voltage-polarized elements. This option is for indication only.
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